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Abstract:

The present invention relates to the field of digital pathology and in
particular to whole slide scanners. Autofocus imaging can be performed by
sampling a first number of pixels of a primary image sensor and sampling
a second number of pixels of an autofocus image sensor, wherein the
second number is between one quarter and three quarters of the first
number. Thus, continuous autofocus for rapid light scanning may be
provided based on an additional image sensor that is tilted with respect
to the optical axis.

Claims:

1. Autofocus imaging system for a microscope, the autofocus imaging
system comprising: a primary image sensor arrangement comprising a
primary image sensor (24) for acquiring primary image data of an object
of interest (4); an autofocus image sensor (26) for acquiring autofocus
image data of an oblique section of the object of interest (4); wherein
the primary image sensor (24) is configured for sampling a first number
of pixels per distance in object space; wherein the autofocus image
sensor (26) is configured for sampling a second number of pixels per
distance in object space; wherein the second number is between one
quarter and three quarters of the first number.

2. The autofocus imaging system of claim 1, wherein the second number is
half of the first number.

3. The autofocus imaging system of claim 1, wherein a resolution of the
primary image sensor is λ/(2NA); and wherein a resolution of the
autofocus image sensor is λ/(NA).

4. The autofocus imaging system of claim 1, wherein the autofocus image
sensor (26) is tilted with respect to an optical axis (31) of the primary
image sensor (24).

5. The autofocus imaging system of claim 1, wherein the autofocus image
sensor (26) is tilted in a scan direction (6) of the autofocus imaging
system.

6. The autofocus imaging system of claim 1, further comprising: a beam
splitter (22) for splitting a beam from the object into a first beam
towards the primary image sensor and a second beam towards the autofocus
image sensor; wherein the a fraction between an intensity of the second
beam and an intensity of the first beam is bigger than
(1-L0/Lmax), wherein Lmax is the maximum number of Time
Delay Integration stages of the primary image sensor.

7. The autofocus imaging system of claim 1, wherein the autofocus image
sensor (24) is configured for acquiring the autofocus image data at a
light frequency outside the frequency of the visible spectrum.

8. The autofocus imaging system of claim 1, wherein the autofocus imaging
system (500) is configured for dark field illumination of the autofocus
image sensor.

9. The autofocus imaging system of claim 1, wherein the primary image
sensor arrangement further comprises a second primary image sensor (32)
and a third primary image sensor (33); wherein each of the primary image
sensors of the primary image sensor arrangement is configured for
detecting light of a different wavelength.

10. The autofocus imaging system of claim 1, wherein the primary image
sensor (24) is a line sensor; and wherein the autofocus image sensor (24)
is a two-dimensional sensor.

11. The autofocus imaging system of claim 1, wherein the primary image
sensor and the autofocus image sensor share a same sensing area.

12. A microscope (802) comprising an autofocus imaging system (500) of
claim 1.

13. A method for autofocus imaging of a microscope (802), the method
comprising the following steps: acquiring primary image data of an object
of interest by a primary image sensor (24) of a primary image sensor
arrangement; acquiring autofocus image data of an oblique section of the
object of interest by an autofocus image sensor (26); sampling a first
number of pixels per distance in object space, the first number of pixels
being pixels of the primary image sensor (24); sampling a second number
of pixels per distance in object space, the second number of pixels being
pixels of the autofocus image sensor (26); wherein the second number is
between one quarter and three quarters of the first number.

14. A computer-readable medium, in which a computer program for autofocus
imaging of a microscope is stored which, when executed by a processor
(800) of the microscope (802), causes the processor (800) to carry out
the steps of: acquiring primary image data of an object of interest from
a primary image sensor (24) of a primary image sensor arrangement;
acquiring autofocus image data of an oblique section of the object of
interest from an autofocus image sensor (26); sampling a first number of
pixels per distance in object space, the first number of pixels being
pixels of the primary image sensor (24); sampling a second number of
pixels per distance in object space, the second number of pixels being
pixels of the autofocus image sensor (26); wherein the second number is
between one quarter and three quarters of the first number.

15. A program element for autofocus imaging of a microscope, which, when
being executed by a processor (800) of the microscope (802), causes the
processor to carry out the steps of: acquiring primary image data of an
object of interest from a primary image sensor (24) of a primary image
sensor arrangement; acquiring autofocus image data of an oblique section
of the object of interest from an autofocus image sensor (26); sampling a
first number of pixels per distance in object space, the first number of
pixels being pixels of the primary image sensor (24); sampling a second
number of pixels per distance in object space, the second number of
pixels being pixels of the autofocus image sensor (26); wherein the
second number is between one quarter and three quarters of the first
number.

Description:

FIELD OF THE INVENTION

[0001] The present invention relates to the field of digital pathology,
notably. In particular, the present invention relates to an autofocus
imaging system for a microscope, a microscope comprising an autofocus
imaging system, a method for autofocus imaging of a microscope, a
computer-readable medium and a program element.

BACKGROUND OF THE INVENTION

[0002] In digital pathology and particular in the case of whole slide
scanning, specimens are sliced and imaged for analysis purposes as well
as teaching purposes. Line sensors may be used for scanning a whole
tissue slide. These slide scanners may perform a continuous mechanical
scanning, thereby reducing stitching problems and allowing for the use of
so-called time delay integration (TDI) line sensors in order to
accommodate for low brightness of the illumination.

[0003] For focusing focus maps may be used. Before the actual scanning the
optimum focus position is determined at a number of positions on the
slide. This results in a "focus map". This procedure may be necessary
because the axial position of the tissue layer may vary with several
micrometers across the slide, as may be seen in FIG. 1. The variation of
the tissue layer may thus be more than the focal depth of the microscope
objective. During scanning the focus position of the objection is set on
a trajectory that interpolates between the measured optimum focus
settings on the selected measurement locations. This procedure may be
both prone to errors and be also time-consuming, thereby limiting the
throughput of the system.

[0004] WO 2005/010495 A2 describes a system and a method for generating
digital images of a microscope slide, the microscope comprising a main
camera and a focus camera which is tilted with respect to the optical
axis.

SUMMARY OF THE INVENTION

[0005] However, the performance of the autofocus function may be
insufficient.

[0006] It may be desirable to have an autofocus imaging system with
improved performance.

[0007] According to a first aspect of the invention an autofocus imaging
system for a microscope is provided, which comprises a primary image
sensor and an autofocus image sensor. The primary image sensor is adapted
for acquiring primary image data of an object of interest, such as a
tissue slide. The autofocus image sensor is adapted for acquiring
autofocus image data of an oblique section of the object of interest. The
primary image sensor is further adapted for sampling a first number of
pixels per distance in object space and the autofocus image sensor is
further adapted for sampling a second number of pixels per distance in
object space, wherein the second number is between one quarter and three
quarters of the first number.

[0008] In other words, the autofocus image sensor samples a smaller number
of pixels per distance in object space than the primary image sensor. By
sampling a smaller number of pixels, the computational load and also the
sampling time may be reduced. Furthermore, by sampling not less than one
quarter of the pixels which are sampled by the primary image sensor the
quality of the autofocus sensor signal may be optimized.

[0009] According to an exemplary embodiment the second number is half of
the first number. In other words, the autofocus image sensor samples half
the numbers of pixels per distance in object space in the primary image
sensor.

[0010] The primary image sensor assembly may comprise one line sensor or
may comprise more than one line sensor, for example three or even more
line sensors. Each line sensor may detect a different wavelength or
wavelength range. For example, one line sensor may detect green light, a
second red light and a third line sensor may detect blue light (only).

[0011] According to another exemplary embodiment the autofocus image
sensor is tilted with respect to an optical axis of radiation from the
object of interest towards the autofocus image sensor, e.g. tilted with
respect to an optical axis of the primary image sensor. In this way the
position of the tissue layer on the sensor is a measure for the amount of
defocus.

[0012] According to another exemplary embodiment the autofocus image
sensor is adapted for acquiring the autofocus image data at a light
frequency outside the frequency of the visible spectrum.

[0013] According to another exemplary embodiment the autofocus imaging
system is adapted for dark field illumination of the autofocus image
sensor.

[0014] In other words, the object of interest may be illuminated with a
beam comprising a set of directions of propagation, such that the angle
of these directions of propagation is larger than the angle sub-tended by
the detection aperture of the autofocus imaging sensor. In this way light
reflected from various surfaces (air, cover slip, cover slip-tissue
layer, tissue layer-slide, slide-air) may not end up at the autofocus
image sensor. In fact, all low object spatial frequencies may be blocked
and only signal emanating from the tissue (which has sufficiently high
spatial frequencies) may be detected at the autofocus image sensor. This
may improve the robustness and accuracy that the axial position of the
tissue layer can be measured.

[0015] According to a second aspect of the invention a microscope
comprising an above and below described imaging system is provided.

[0016] According to an exemplary embodiment of the invention, the
microscope is adapted as a slide scanner for digital pathology.

[0017] According to another aspect of the invention a method for autofocus
imaging of a microscope is provided, in which primary image data of an
object of interest is acquired by a primary image sensor, autofocus image
data of an oblique section of the object of interest is acquired by an
autofocus image sensor, a first number of pixels per distance in object
space are sampled, the first number of pixels being pixels of the primary
image sensor, and a second number of pixels per distance in object space
is sampled, the second number of pixels being pixels of the autofocus
image sensor. The second number is between one quarter and three quarters
of the first number.

[0018] According to another aspect of the invention a computer-readable
medium is provided, in which a computer program for autofocus imaging of
a microscope is stored which, when executed by a processor of a
microscope, causes the processor to carry out the above and/or below
described method steps.

[0019] Furthermore, according to another aspect of the invention, a
program element for autofocus imaging of a microscope is provided, which,
when being executed by a processor of a microscope, causes the processor
to carry out the above and/or below described method steps.

[0020] A computer-readable medium may be a floppy disk, a hard disk, a CD,
a DVD, an USB (Universal Serial Bus) storage device, a RAM (Random Access
Memory), a ROM (Read Only Memory) and an EPROM (Erasable Programmable
Read Only Memory). A computer-readable medium may also be a data
communication network, for example the Internet, which allows downloading
a program code.

[0021] It may be seen as a gist of an exemplary embodiment of the present
invention, that the autofocus imaging sensor, which may be a
two-dimensional sensor, samples a smaller number of pixels per distance
in object space and the primary sensor, which may be a line sensor or
which may comprise more than one line sensors. For example, the autofocus
sensor samples half the number of pixels than the primary sensor.

[0022] These and other aspects of the invention will be apparent from and
elucidated with reference to the embodiments described hereinafter.

[0023] Exemplary embodiments of the present invention will now be
described in the following, with respect to the following drawings.

[0028]FIG. 5 shows a microscope with an autofocus imaging system
according to an exemplary embodiment of the invention.

[0029]FIG. 6 shows a microscope with an autofocus imaging system
according to another exemplary embodiment of the invention.

[0030]FIG. 7 shows a microscope with an autofocus imaging system
according to another exemplary embodiment of the invention.

[0031]FIG. 8 shows a microscope system according to an exemplary
embodiment of the invention.

[0032]FIG. 9 shows a flow-chart of a method according to an exemplary
embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0033] The illustration in the drawings is schematically. In different
drawings, similar or identical elements are provided with the same
reference numerals.

[0034] In the following, the character prime (') associated to a symbol
will mean that the image space is considered (e.g. sensor reference)
while a symbol without prime character will mean that the object space is
considered (typically the sample reference). For example, when the angle
Beta prime (β') will be used in this decription, a rotation in image
space, and, as will be described more specifically, a rotation of the
physical sensor, will be indicated. Also, an angle Beta (β without
prima) will indicate a rotation in object space, and as will be described
more specifically a rotation of an oblique cross section of the sample
that is imaged by the autofocus sensor.

[0035] FIG. 1 shows a schematic cross-section of a tissue slide assembly,
comprising a microscope slide 1, having a typical thickness of 1 mm, a
cover slip 2, with a typical thickness of 0.17 mm, a mounting medium 3
for fixing and sealing off a tissue layer 4. The tissue layer is
typically around 5 μm thick, the mounting layer includes the tissue
layer and is typically 10-15 μm thick. The mounting medium may be
applied to the slide with tissue layer in liquid form before a cover slip
is attached to the slide, subsequently the mounting liquid solidifies,
thus mechanically fixing the tissue layer and sealing it off from the
outside environment in order to provide stability against deterioration.
The axial position of the tissue layer may vary within several μm
across the slide.

[0036] For providing an optimum resolution during scanning the focus may
have to be adjusted continuously, since the axial position of the tissue
layer varies.

[0037] An alternative for the use of the "focus map"-method is the use of
a continuous autofocus system, i.e. an additional system that
continuously measures the optimum focus position and adapts the axial
position of the objective lens during the actual scan for acquiring the
digital image. The autofocus system may be based on optimizing the
contrast in the obtained image. A variety of matrix may be used for
contrast optimization. However, the sine of the focus error (above or
below focus) can not be determined in this manner, i.e. the focus error
signal is not polar. This may be disadvantageous for a continuous
autofocus system that needs permanent updates on the optimum focus
setting.

[0038] The autofocus system may use the line reflected at a reference
surface at or near the object plane, such as in optical disks. However, a
drawback of this method when applied to tissue slides may be that the
relevant interface (between microscope slide and tissue layer and between
tissue layer and cover slip) may have a low reflectance and that the
reflection signal is distorted by scattering arising from the nearby
tissue layer, thus comprising robustness.

[0039] A good alternative is the use of an additional sensor that is
tilted with respect to the optical axis. This autofocus image sensor
makes an image of an oblique section of the object, as depicted in FIG.
2. This section may cut through the tissue layer at some point depending
on the axial position of the tissue layer or relative to the focal plane
of the objective lens. In this way the position of the tissue layer on
the sensor is a measure for the amount of defocus. For more details on
these aspects, the reader may refer to the European patent application
N°09306350.

[0040] As can be seen from FIG. 2 the tilted autofocus image sensor makes
an image of an oblique cross-section 5 of the tissue slide assembly. The
tilt is in the scanning direction 6. The sensor has Nx pixels and
samples the object in the scan direction with Δx per pixel and in
the axial direction with Δz per pixel.

[0041] For example, the autofocus imaging system operates using
wavelengths outside the visible spectrum so as not to spoil the white
light imaging of the tissue layer. For example, the autofocus system
operates using wavelengths on the infrared side of the visible spectrum,
because ultraviolet radiation may damage the tissue and may require more
complicated and/or expensive optical components than infrared radiation.

[0042] In an exemplary embodiment, the additional autofocus image may be
provided by using a so-called dark field illumination. Hereby, the sample
is illuminated with a beam comprising a set of directions of propagation,
as already described above.

[0043] A problem may arise if the tilted autofocus sensor is combined with
a time delay integration (TDI) line sensor (primary image sensor) for
high throughput imaging. Such a TDI-line sensor records each object pixel
L times, where the number of stages L can be typically up to 128. This
has the effect that the total integration time, and hence signal level,
increases by a factor L compared to a conventional single line sensor.
This is used to increase the scanning speed of the system.

[0044] A reasonable starting point in the design of such a system may
entail having a resolution Raf of the autofocus sensor approximately
equal to the resolution Rim of the (TDI-based) image sensor in order
to be able to test the same level of sharpness in the image. The novel
insight of the inventors is that this implies a problem with signal level
on the autofocus image sensor as will be apparent from the following
considerations. Taking a linear scan speed v the line rate of the image
sensor is:

1 T im = 2 v R im ( 1.1 ) ##EQU00001##

[0045] (NB: pixel size=half the resolution) making the total integration
time LTim. In order to prevent motion blur the autofocus sensor must
have a shutter such that the collection time is:

T af = R af 2 v ( 1.2 ) ##EQU00002##

[0046] The beam after the objective lens is split in two parts by a beam
splitter, a fraction η is directed towards the autofocus sensor, and
a fraction 1-η towards the image sensor. If the slide is illuminated
with an intensity B (incident power per area), then the signal level at
the image sensor and at the autofocus image sensor are given by:

[0047] where ηim is the image sensor (quantum) efficiency and
ηaf is the autofocus sensor (quantum) efficiency. These sensor
efficiencies may be assumed to be approximately equal. The ratio of the
two is:

[0048] If Lo=(1-η)L is the number of stages that would be needed
if no autofocus sensor was used and taking Raf≈Rim and
ηaf≈ηim it follows that:

I af I im ≈ η L 0 1 ( 1.5 )
##EQU00005##

[0049] Clearly, the signal level at the autofocus sensor is much smaller
than the signal level at the image sensor. As a consequence, the
autofocus sensor signal will be relatively noisy, which compromises the
accuracy of the focus error signal.

[0050] There may be a significant redundancy in the resolution
requirements on the autofocus sensor compared to the resolution
requirements on the image sensor. This insight follows from the study of
the effect of defocus on the so-called Modulation Transfer Function
(MTF), which is the ratio of the modulation in the image of a periodic
object and the modulation in the object itself as a function of spatial
frequency (the inverse of the period p). The MTF as a function of defocus
for the simplified 1D-case with equal condenser and objective NA is given
by:

MTF = sin c ( 2 πβ q ( 2 - q )
) ( 1 - q 2 ) ( 1.6 ) ##EQU00006##

[0051] with sinc(x)=sin(x)/x, q=λ/pNA is the normalized spatial
frequency, and β--ΔzNA2/2nλ is a defocus parameter.
FIG. 3 shows the MTF for the nominal in-focus situation 301 and for a
case with defocus 302. The x-axis 303 depicts the normalized spatial
frequency and the y-axis 304 depicts the MTF values.

[0052]FIG. 4 shows the ratio 401 of the two MTF-functions 301, 302. The
y-axis 403 depicts the MTF ratio; a minimum 402 can be observed the x
value equal to 1. Both MTF-functions show a cut-off at 2NA/λ, (this
is the so-called `diffraction limit`), which is the ultimate resolution
limit for a conventional microscope. The ratio of the two MTF-functions
shows a dip for the middle spatial frequencies. From this analysis we may
conclude that:

[0053] The resolution of the primary image sensor is
preferably determined by the so-called Nyquist-criterion for the
2NA/λ, spatial frequency cut-off to Rim=λ/2NA (so pixel
size Mimλ/4NA, with Mim the magnification from object to
image sensor).

[0054] The resolution of the autofocus sensor is
preferably determined by the maximum in defocus sensitivity to half the
spatial frequency cut-off to Raf=λ/NA (so pixel size
Mafλ/2NA, with Maf the magnification from object to
autofocus sensor).

[0055] According to an exemplary embodiment the autofocus image sensor
sampling (pixels/m in object space) is selected 3/4 to 1/4 or, e.g., at
least a factor two smaller than the image sensor sampling. This gives a
good compromise between defocus sensitivity and autofocus to image signal
ratio. Preferably, the beam splitter fraction η is adapted such that
the autofocus sensor signal is sufficiently high compared to the image
sensor signal. Preferably, the parameter settings are such that the
TDI-based line sensor has sufficient redundancy to maintain a
sufficiently high image sensor signal, i.e. η>1-L0/Lmax,
where Lmax is the maximum number of TDI-stages.

[0056] This is different from the implementation of a secondary camera
autofocus method based on the addition of a dedicated image sensor for
autofocus, which is not tilted with respect to the plane in the object
that is being imaged, and where the difference in resolution
(specifically a lower resolution of the autofocus sensor) serves the sole
purpose of increasing the speed of the autofocus sensor with respect to
the primary image capturing sensor. Also the reduction in pixel count is
described for several embodiments as a factor of at least 3, and a factor
of at least 10. As is seen in the minimum in the bottom graph of FIG. 3,
the inventors specifically found an optimum at a reduction in the
resolution of exactly 2. Although a practical range for a second
embodiment would be a range between 4/3 and a factor 4.

[0057] The depth range Δztot of the autofocus system must be
sufficiently large for realistic settings of other parameters. The
autofocus image sensor has Nx pixels in the scan direction, with
pixel size b. The sensor is tilted over an angle β' so that the
lateral and axial sampling is given by:

Δx'=b cos β'

Δz'=b sin β'

[0058] The lateral and axial sampling at the object (the tissue slide) is
given by:

Δx=Δx'/M

Δz=nΔz'/M 2

[0059] where M is the magnification and n the refractive index of the
object. The axial sampling at the object now follows as:

[0061] Table 1 shows an example of parameter settings according to the
invention. In this example the autofocus resolution is 2×0.9 μm,
whereas the image resolution is preferably about 2×0.25 μm
(taking a 20×/NA0.75 microscope objective).

[0062] As a non limitative example, FIG. 5 shows part of a microscope and
in particular the imaging branch of the light path. An embodiment for
epi-mode dark field illumination is shown in FIG. 6.

[0063] The light passing through the slide 1 and the cover slip 2 (and
tissue layer 4, not shown) is captured by the objective lens 20 with the
back aperture 21, wherein the unscattered beams are blocked. A colour
splitter 22 splits off the white light which is imaged by a tube lens 23
onto the image sensor arrangement, which may comprise a first, a second
and a third primary image sensor 24, 32, 33, which may be adapted in the
form of line sensors, 24 for generating the digital tissue image. The
infrared light is imaged by a second tube lens 25 onto the autofocus
image sensor 26, which is tilted with respect to the optical axis 31 of
radiation from the object of interest towards the autofocus image sensor
26. In the context of this disclosure "tilted with respect to the optical
axis of the primary image sensor" means that the radiation from the
object of interest which impinges on the autofocus image sensor does not
impinge on the autofocus image sensor perpendicularly. However, the
radiation which travels from the object of interest towards the primary
image sensor may impinge perpendicularly on the primary image sensor,
although this is not required already described herein above. Rays
scattered by the tissue can pass through the aperture 21 and are imaged
onto the autofocus image sensor 26.

[0064]FIG. 6 shows an optical layout for epi-mode dark field illumination
of a microscope with an autofocus imaging system 500 having a laser diode
14, the illumination being integrated with the imaging branch. Two
crossed gratings 15 are arranged after the laser diode 14 for generating
diffraction orders, for example a 0th diffraction order S'0, a
+1st order S'+1 and a -1st order S'-1. Still further,
a field stop 16 is arranged close to the gratings 15 for limiting the
width of the dark field illumination beams, and a collimator lens 17 the
collimates the light from the laser diode 14.

[0065] A polarizing beam splitter 28 is provided to split the beam after
it has passed the collimator lens 17. Furthermore, the microscope
comprises quarter-wave plate 29. Both elements 28 and 29 take care of
directing the beam originating from the laser towards the objective lens
and erecting the scattered light originating from the tissue towards the
autofocus image sensor.

[0066]FIG. 7 shows an optical layout for multi-spot illumination of a
microscope 500, which illumination is integrated with the imaging branch.
The lens 17 collimates the beam which is incident on a spot generator for
generating an array of spots 30. By tilting the whole assembly, the spot
array can be tilted so that the resulting incident spot array and the
slide is tilted as well. The spot generator 29 generates an array of
low-NA beams, which can pass the beam splitter 27 without introducing
significant aberrations.

[0067] In the embodiment of FIG. 7 an array of spots is used to illuminate
the oblique section 5 that is imaged by the autofocus image sensor. The
spots that are focused on the tissue may experience time-dependent
scattering as the absorption and refractive index of the region into
which the spot is focused changes with scanning. By examining the
time-dependence of the spots imaged on the autofocus image sensor the
axial position of a tissue layer may be located. Namely, close to focus
the high resolution information is visible, away from focus this is
blurred. As a consequence, the signal variations on a comparatively small
time scale may be maximum when the tissue layer coincides with the focal
plane.

[0068]FIG. 8 shows a microscope system 802 comprising a microscope with
an autofocus imaging system 500 connected to a processor or processing
unit 800 which is connected to a user interface 801, such as a computer.

[0069]FIG. 9 shows a flow-chart of a method according to an exemplary
embodiment. In step 901 primary and secondary, i.e. autofocus image data
of an object of interest are acquired by a primary image sensor and an
autofocus image sensor, respectively. In step 902 the pixels of the
primary image sensor are sampled. In step 903 (which can be before, after
or at the same time as step 902) a certain number of pixels per distance
in object space of the autofocus image sensor is sampled. This number is
smaller than the sampled number of pixels of the primary image sensor.
Then, in step 904, the focus of the microscope is adjusted based on the
sampling.

[0070] Thus, the focus of the primary image sensor may be adjusted
automatically. In another embodiment of the invention, the principles of
the present invention may be advantageously applied to a sensor which the
applicant of the present invention has already proposed under European
patent application N°09306350, and which is hereby incorporated by
reference.

[0071] As a result, according to this embodiment the primary image sensor
and the autofocus image sensor may share a same sensing area. In other
words, the primary image sensor and the autofocus image sensor may
together form a unique sensor with a sensing area (typically formed of
pixels) that is both used for autofocus and for image acquisition.

[0072] According to this embodiment, the larger autofocus pixels may be
either actual physical pixels located next to or intermixed in the array
or arrays of primary image pixels, or the autofocus pixels may be virtual
pixels obtained by combining two or more of the primary image pixels into
a larger virtual autofocus pixel. Such a combination may be done on the
sensor itself, or in a separate processing unit.

[0073] The described autofocus system finds application in digital
pathology and other fields of rapid micro scanning

[0074] While the invention has been illustrated and described in detail in
the drawings and foregoing description, such illustration and description
are to be considered illustrative or exemplary and not restrictive; the
invention is not limited to the disclosed embodiments. Other variations
to the disclosed embodiments can be understood and effected by those
skilled in the art and practising the claimed invention, from a study of
the drawings, the disclosure, and the appended claims. In the claims, the
word "comprising" does not exclude other elements or steps, and the
indefinite article "a" or "an" does not exclude a plurality. The mere
fact that certain measures are recited in mutually different dependent
claims does not indicate that a combination of these measures cannot be
used to advantage. Any reference signs in the claims should not be
construed as limiting the scope.